As government controls on internal combustion engine emissions become more strict, there arises an increased concern for more effective operation of the systems that reduce harmful engine emissions. Attention has been turned to conventional catalytic converters and their operation. It is known that conserving the residual heat of exhaust gases of an internal combustion engine so that the downstream catalytic converter may operate with higher efficiency and effectiveness reduces the emission levels of the engine. More specifically, in order to meet new emission standards, it is necessary to get the exhaust system catalyst to "light off" very quickly because a high percentage of total emissions occur during cold start, before the catalysts are active. Solutions to this problem must be inherently durable, yet not introduce any additional engineering or manufacturing problems into production of the automobile.
Various solutions have been advocated which include cast-in-place heat insulating type liners. Insertable liners may be added independently of the fabrication of the exhaust manifold. Coatings may be applied directly to the prefabricated engine components including asbestos and other ceramic materials. But cast-in-place type liners are affected by shrinkage and solidification of the cast metal around the liner and have led to localized peeling and/or separation of the cast-in-place liners from the engine component which eventually leads to damage, leaks, and inadequate insulation. Insertable type liners often develop sealing difficulties.
The disadvantages of coatings are specifically related to their fragile nature. This is particularly amplified when the cast housing is subjected to mechanical or chemical treatments, which may tend to fracture or chip the coatings. Coating systems also require multiple manufacturing steps, which result in increased manufacturing costs. Further, thin coatings are ineffective because the minimal thickness does not produce a thermal resistance sufficient to overcome the increased heat transfer caused by the increased roughness of the thin coating surface.
One further solution which has been suggested, is to lower the thermal mass of the exhaust manifold and exhaust pipe in front of the catalytic converter. This solution requires reducing the thermal mass of the exterior manifold by reducing the thickness of the manifold and/or exhaust pipes. In this system, a thin walled inner pipe is enclosed in a thicker walled outer pipe which is provided for overall structural integrity. The thicker walled pipe provides a cooler outer wall surface adjacent to the other engine and body components due to the air gap.
The thickness of the inner pipe is normally limited to dimensions that can be fabricated and still retain sufficient structural integrity. As the inner and outer pipes operate at different temperatures, a means for allowing the differential thermal expansion is often included in the above discussed inner and outer pipe designs. The inner and outer pipe design incorporates a sliding joint or a convoluted pipe design which is fairly complex. The sliding joint must be carefully designed to avoid the high probability of a fracture or failure in use over an extended period of time.
U.S. Pat. No. 4,890,663 issued to Yarahadi discloses a method for producing a metallic component provided with a ceramic lining. A first ceramic layer is applied to a mold, a second sliding layer is applied to the first ceramic layer and a second ceramic layer is thereby divided by joints into individual zones and is applied to the sliding layer. Lastly, the second ceramic layer is coated with a metal to form a finished component.
U.S. Pat. No. 4,884,400 issued to Tanaka, et al. discloses an exhaust manifold for an internal combustion engine which comprises an outer body of aluminum having a configuration corresponding to an exhaust manifold and an insulating layer of ceramic fiber disposed on the inner surface of the outer body for protecting the outer body from heated exhaust gas. A protector is incorporated within the insulating layer in a manner to maintain the disposition and configuration of the insulating layer.